docs: def sub(self, index):
          """Return a sub-attribute"""
          index = np.asarray(index)
          if index.dtype == 'bool':
              index = index.nonzero()[0]
          
          if self.size < len(index):
              raise ValueError('Can\'t subset "{}": index ({}) is bigger than the number of elements ({})'.format(self.name, len(index), self.size))
          
          inst = self.copy()
          size = len(index)
          inst.empty(size)
          
          if len(index) > 0:
              inst.value = self.value.take(index, axis=0)
          
          return inst